Hi @rpsimon,
@dbrace definitely provided you with a couple different options. There may be a few other useful admin reports (the Zero Activity comes to mind if you're looking to find students who have never visited a course from day one), but it's hard to make a solid recommendation without knowing more about your needs...
You mention "logged in to their courses", which is actually a combination of things... A user logs in to their account, but that doesn't necessarily mean they visited any particular course (and sometimes users rarely need to log in again, especially with the mobile apps). Course access may me more of what you're after, or even perhaps participation (where a student actually submits work). While these are all related things, they are distinct, so knowing what you need is very important.
Please let us know any additional details you can so it's easier to determine if one of the already suggested reports fits the bill, or if something different would be needed.
-Chris